1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a transflective liquid crystal display device.
2. Description of the Related Art
Liquid crystal display devices, being thin, light-weight, and low in power consumption, are used as a display monitor in a wide array of electronic devices including notebook personal computers, portable information terminals, cellular phones, and digital cameras. Unlike CRT displays and plasma displays, liquid crystal display devices do not emit light themselves but utilize external light to display images and other information by controlling the amount of incident light. Liquid crystal display devices can display images in multiple colors when equipped with a color filter, which has multiple colors, as light control elements.
Liquid crystal display devices of this type have a pair of substrates (hereinafter also referred to as “first substrate and second substrate”) between which a liquid crystal layer is held to form liquid crystal cells, and turn an electronic latent image into a visible image by applying an electric field to the liquid crystal layer and thus controlling the molecular orientation of a liquid crystal composition that constitutes the liquid crystal layer. Liquid crystal display devices are classified into passive matrix type and active matrix type by driving method. Currently, active matrix liquid crystal display devices are predominant because of their capability to display high-definition images at high speed. In active matrix liquid crystal display devices, the first substrate has active elements (switching elements), typically, thin-film transistors, for selecting pixels, and the second substrate has a color filter painted in three colors for color display.
Many of cellular phones and other similar information terminals called mobile devices employ transflective liquid crystal display devices, which can use selectively or simultaneously a reflective method where an image is displayed with light entering from the viewer's side and a transmissive method where an image is displayed with transmitted light of light incident on the opposite side to the viewer's side.
Not being self-luminous, liquid crystal display devices need to visualize an electronic latent image with lighting that uses visible light which then exits as image light to the viewer's side. A method in which natural light (ambient light) or other illumination light is cast from the viewer's side is called a reflective method, whereas a method in which illumination light is cast from the opposite side to the viewer's side is called a transmissive method. Liquid crystal display devices that can handle the method of casting illumination light from the viewer's side and the method of casting illumination light from the opposite side to the viewer's side both are called transflective (half-transmissive, half-reflective) liquid crystal display devices.
Transflective liquid crystal display devices which have the properties of transmissive LCDs and reflective LCDs both provide a good visibility in a diversity of environments from the dark indoors to the bright outdoors. Because of their good outdoor visibility, transflective liquid crystal display devices are often employed in the above mobile devices. It is common for recent mobile devices to have a camera function and a function of browsing the Web which holds numerous pieces of image data and, as a result, high image quality is demanded from liquid crystal display devices for mobile devices.
Conventional transflective liquid crystal display devices make the transmissive display method and the reflective display method compatible by attaching a retardation film between a polarizing plate and a substrate of a liquid crystal cell such that the optical axis of the retardation film runs in a direction different from the optical axis direction of the polarizing plate. With retardation films which have a fluctuating degree of retardation without exception, it is difficult to raise the contrast of light transmission. To address this difficulty, transflective liquid crystal display devices that do not need a retardation film have been proposed in, for example, JP 2005-338264 A and JP 2007-47734 A.
Liquid crystal display devices disclosed in these documents both employ, as a liquid crystal display method, in-plane switching (hereinafter abbreviated as IPS) in which liquid crystal molecules are driven with a mainly lateral electric field generated between a pixel electrode and a common electrode which are formed on the same substrate. IPS does not need a retardation film whose optical axis runs in a direction different from the optical axis direction of the polarizing plate, and therefore is easy to raise the light transmission contrast. In addition to the above documents, U.S. Pat. No. 6,122,103 B is given as a related art relevant to the present invention.
The liquid crystal display devices disclosed in JP 2005-338264 A and JP 2007-47734 A both have such a feature that liquid crystal molecules are aligned in different directions in a transmissive area and a reflective area. In the following description, aligning liquid crystal molecules in different directions in a transmissive area and a reflective area, or any other state in which the direction of the liquid crystal alignment differs among multiple regions within a pixel, is called alignment-segmentation.
One of processes that have been proposed to accomplish alignment-segmentation is to perform a partial alignment process such as mask rubbing region by region. However, performing a partial alignment process for a transmissive area and a reflective area separately could produce regions that receive neither the alignment process for a transmissive area nor the alignment process for a reflective area if the substrate is not positioned properly during a switch between the former and the latter. Regions that receive no alignment process contain misaligned liquid crystal molecules, which can affect displayed images.